The invention relates to a plasma generator and a plasma torch employing such a plasma generator.
Plasma spraying is used to form a coating on a substrate. It generally consists in producing an electric arc, in blowing a plasmagen gas through this electric arc so as to generate a very high-temperature, high-speed plasma flux, then in injecting into this plasma flux particles so as to spray them onto the substrate. The particles melt, at least partially, in the plasma and can thus adhere well to one another and to the substrate when they cool. This technique may thus be used to coat the surface of a substrate made of a metal, ceramic, cermet, polymer, organic material or a composite, in particular a composite comprising an organic matrix. This technique is especially used to coat parts having various shapes that have for example planar or axisymmetric geometries, especially cylindrical geometries, or complex geometries, these parts possibly having various sizes—the only limit being access by the jet of particles. The aim may be, for example, to provide a substrate with a surface functionality such as wear resistance, or to modify the friction coefficient, the thermal barrier or the electrical insulation.
This technique may also be used to manufacture bulk parts, by way of a technique called “plasma forming”. By virtue of this technique it is thus possible to apply a coating a number of millimeters in thickness, even more than 10 mm in thickness.
Plasma torches, or plasmatrons, are for example described in WO 96/18283, U.S. Pat. Nos. 5,406,046, 5,332,885, WO 01/05198 or WO 95/35647 or U.S. Pat. No. 5,420,391.
The performance parameters of a plasma torch for industrial purposes may be said to be the following:                high spray productivity, the spray productivity being defined as the amount of material deposited per unit time;        high deposition efficiency, the deposition efficiency being defined as the ratio, in wt %, between the amount of material deposited and the amount of material injected into the plasma flux;        maximum coating quality, and in particular the ability to produce a uniform and reproducible coating, including with a high material flow rate;        minimum energy consumption;        lowest possible maintenance time with the highest possible time interval between two consecutive maintenance operations; and        reduced contamination via loss of the cathode material.        
One object of the exemplary embodiments is to provide a plasma torch that at least partially meets these criteria.